Laser spectroscopy takes advantage of the laser lines produced by lead salt, III-V compound materials, and other substances to probe the wavelength region between one and 30 micrometers, the so-called infrared fingerprint region, in which the characteristic vibration-rotation lines for most molecules are located. The laser is tuned to the appropriate frequency corresponding to the absorption feature of a species of interest, and passed through a sample gas to be probed. The sample gas absorbs the emitted radiation in characteristic ways dependent on the presence therein of the species of interest. It is thus possible to analyze trace gases with a very high resolution such as for air pollution monitoring, chemical laser systems diagnosis, among others.
In practical laser spectrometry systems heretofore, the laser absorption is sensitively dependent on the prevailing temperature, optical, and electronic conditions of the spectrometry instrument. In the heretofore known dual-beam laser spectrometers, as that disclosed, for example, in commonly assigned U.S. Pat. No. 4,410,273 incorporated herein by reference, the beam dividing optics, the separate sample and reference cells, the different sample and reference beams, and the different detectors provided for the reference absorption and sample absorption measurements each introduce and combine to introduce variables that must be painstakingly compensated. Otherwise, changes in intensity of the measured signal may not reflect absorption by the species of interest but rather differential drift of the different sample and reference detectors, variable absorbency due to the differences in temperature between the reference and sample cells, and differences in the sample and reference beams introduced by the optics. The compensation procedures that must be employed to neutralize such effects not only are bothersome and time consuming, but notwithstanding such procedures, there still is present some element of uncertainty in the resolution with which the measurements are taken.
In addition to the compensation of the effects of the instrument, the measured data must be normalized to the intensity of the tunable laser source so that from that reference the true concentration of gas may be calculated. In the heretofore known laser spectrometers, normalization was typically required either prior to or after laser absorption measurements. But, among other disadvantages, the normalization procedure would often require separate standard sample measurements and the consumption of specially prepared and expensive standard normalization samples.